1. Field of the Invention
The invention relates to an image forming apparatus adopting the electrophotographic process, the electrostatic recording process or the like, and particularly to an image forming apparatus using a transfer material transporting member and a transfer medium as an intermediate transfer member, and having the function of automatically correcting image misregister during image-on-image formation.
2. Description of Related Art
There has heretofore been proposed an image forming apparatus in which there are disposed a plurality of image forming means each for applying a laser beam modulated in conformity with recording information to a photosensitive drum which is an image bearing member or light emitted by a light emitting element such as an LED (light emitting diode), developing an electrostatic latent image formed on the photosensitive drum by the electrophotographic process and transferring a toner image of each color to transfer paper or an intermediate transfer belt, and the toner images of respective colors are multi-transferred on the transfer paper while the transfer paper is sequentially transported to the respective image forming means by a transfer material transporting belt or the toner images of respective colors are multi-transferred on the intermediate transfer belt, thereafter a color image can be formed by a method of collectively transferring the polychromatic toner images primary-transferred to the intermediate transfer belt to the transfer paper.
In the image forming apparatus of this type, there is a case where the positions (registrations) of the respective color images formed on the respective photosensitive drums are not registered with one another on the transfer material to which they are finally multi-transferred, for such reasons as the mechanical mounting errors among the photosensitive drums, the optical path length errors of the respective laser beams, the changes in the optical path and the warp of the LED by the environmental temperature.
Therefore, as shown in FIG. 2 of the accompanying drawings, an image misregister detection pattern 3 formed from each photo-sensitive drum onto the transfer material transporting belt or the intermediate transfer belt 31 which is a transfer medium is read by light sensors 2a, 2b, and the deviation of registration on the photosensitive drum corresponding to each color is detected, and electrical correction is effected on an image signal to be recorded and a turn-back mirror provided in the optical path of the laser beam is driven to thereby effect the correction of any change in the optical path length or any change in the optical path.
Various patterns have been proposed as the image misregister detection pattern 3, and for example, in Japanese Patent Application Laid-Open No. 2000-98810, there is proposed a pattern comprising a first segment disposed with a predetermined angle with respect to a process direction which is the direction of movement of a transfer belt and a second segment disposed axisymmetrically with the first segment with respect to an imaginary line orthogonal to the process direction.
FIG. 2 shows the manner in which the light sensors 2a, 2b detect the image misregister detection pattern 3 on the intermediate transfer belt 31 which is a belt member, and the image misregister detection pattern 3 is read by the light sensors 2a, 2b of an LED 4a which is a light emitting element and a phototransistor 4b which is a light receiving element. These light sensors 2a, 2b are disposed in two sets (2a and 2b) at a predetermined distance therebetween in a direction orthogonal to the process direction, and the image misregister detection pattern 3 is formed so as to pass on the light sensors 2a, 2b. 
As the material of the intermediate transfer belt 31, use is made of a material of which the reflectance for the light (e.g. infrared light) applied from the LED 4a which is a light emitting element in the light sensors 2a, 2b is great as compared with the reflectance of the image misregister detection pattern 3, and by this difference in reflectance, the pattern detection of the image misregister detection pattern 3 is made possible.
FIG. 3 of the accompanying drawings shows a light receiving circuit 17 for reflecting the light applied from the LED 4a to the image misregister detection pattern 3 or the intermediate transfer belt 31, and converting the output signal when the reflected light thereof is received by the phototransistor 4b which is a light receiving element into an electrical signal.
In FIGS. 2 and 3, when a region of the intermediate transfer belt 31 is detected by the light sensors 2a, 2b, the quantity of reflected light is great and therefore a great deal of photocurrent passes through the phototransistor 4b and it is current/voltage-converted by a resistor 5, and is amplified by resistors 6, 7, 8 and an operational amplifier 9.
On the other hand, when the image misregister detection pattern 3 is detected by the light sensors 2a, 2b, the quantity of reflected light is small and therefore, a photocurrent small as compared with that in the region of the intermediate transfer belt 31 passes through the phototransistor 4b, and it is likewise current/voltage-converted by the resistor 5, and is amplified by the resistors 6, 7, 8 and the operational amplifier 9.
FIG. 4 of the accompanying drawings shows a state in which the light receiving circuit 17 has detected the reflected light in the order of a region of the intermediate transfer belt 31xe2x86x92the image misregister detection pattern 3xe2x86x92a region of the intermediate transfer belt 31. In FIG. 4, a threshold level Vt is set intermediately of a transfer belt detection level Va at which the intermediate transfer belt 31 has been detected by the light sensors 2a, 2b and a pattern detection level Vb at which the image misregister detection pattern 3 has been detected.
This threshold level Vt is set by a variable resistor 18 shown in FIG. 3, and by a voltage value outputted from the operational amplifier 9 after the photocurrent passing through the phototransistor 4b has been current/voltage-converted and the voltage value of the threshold level Vt set by the variable resistor 18 being compared with each other by a comparator 19, a pattern detection output 28 shown in FIG. 4 can be created.
Design is made such that the pattern detection output 28 sequentially sent is read, and the deviation of registration is detected from the width, interval or the like of the image misregister detection pattern 3 and electrical correction is effected on an image signal to be recorded and further, the turn-back mirror provided in the optical path of the laser beam is driven to thereby effect the correction of any change in the optical path length or any change in the optical path.
However, when in the aforedescribed example of the conventional art, an unexpected stain, scar or the like occurs on the intermediate transfer belt 31, reflectance is reduced on such portions and the reflected light may not be received in some cases by the phototransistor 4b in the light sensors 2a, 2b. 
In that case, the stain or scar on the intermediate transfer belt 31 is recognized as the detection pattern 3, and the accurate width or interval of the image misregister detection pattern 3 cannot be read, and this has led to the problem that the deviation of registration cannot be corrected accurately.
The present invention solves the above-noted problem and the object thereof is to provides, in a construction for detecting an image misregister detection pattern, and on the basis of the result thereof, effecting the registration correction of image forming means, an image forming apparatus which is free from wrong detection due to the stain, scar or the like of a transfer medium and which precisely reads only the image misregister detection pattern to thereby effect highly accurate correction and realize a high quality of image.
A preferred form of the present invention for achieving the above object is an image forming apparatus comprising:
at least two image forming means;
transferring means for successively superimposing and transferring images formed by the image forming means onto a transfer medium;
pattern forming means for forming an image misregister detection pattern for detecting the positional deviation between the images formed by the image forming means;
pattern detection means for detecting the image misregister detection pattern formed on the transfer medium by the pattern forming means;
pattern position detecting means for detecting the position of the image misregister detection pattern from a detection signal obtained from the pattern detecting means; and
positional deviation correcting means for correcting the positional deviation of at least one of the image forming means on the basis of the result of the detection of the pattern position detecting means;
wherein the pattern positional detecting means detects the time at a point of time whereat the detection signal of the pattern detecting means has been continuously outputted for a predetermined time, and the time at a point of time whereat thereafter the outputting of the detection signal has been stopped, to thereby detect the position of the image misregister detection pattern.